Cyclohexylbenzene can be produced from benzene by the process of hydroalkylation or reductive alkylation. Benzene is heated with hydrogen in the presence of a catalyst such that the benzene undergoes partial hydrogenation to produce cyclohexene, which then alkylates the benzene to form cyclohexylbenzene.
U.S. Pat. Nos. 4,094,918 and 4,177,165 disclose hydroalkylation of aromatic hydrocarbons over catalysts of nickel- and rare earth-treated zeolites and a palladium promoter. Similarly, U.S. Pat. Nos. 4,122,125 and 4,206,082 disclose the use of ruthenium and nickel compounds supported on rare earth-treated zeolites as aromatic hydroalkylation catalysts. The zeolites employed are zeolites X and Y. In addition, U.S. Pat. No. 5,053,571 proposes the use of ruthenium and nickel supported on zeolite beta as an aromatic hydroalkylation catalyst. These processes have disadvantages of low selectivity to cyclohexylbenzene, particularly at economically viable benzene conversion rates, and large quantities of unwanted by-products, particularly cyclohexane and methylcyclopentane.
U.S. Pat. No. 6,037,513 discloses that cyclohexylbenzene selectivity in the hydroalkylation of benzene can be improved by contacting benzene and hydrogen with a bifunctional catalyst having at least one hydrogenation metal and a molecular sieve of the MCM-22 family. The hydrogenation metal is preferably selected from among palladium, ruthenium, nickel, cobalt and mixtures thereof, and the contacting step is conducted at a temperature of about 50° C. to 350° C., a pressure of about 100 to 7000 kPa, benzene to hydrogen molar ratio of about 0.01 to 100 and a WHSV of about 0.01 to 100. The '513 patent discloses that the cyclohexylbenzene can then be oxidized to the corresponding hydroperoxide and the peroxide cleaved to the desired phenol and cyclohexanone.
Although the use of MCM-22 family catalysts has afforded an increase in cyclohexylbenzene selectivity, significant amounts of cyclohexane and methyl cyclopentane are nonetheless produced. The conditions of low temperature (less than 200° C.) and high pressure (greater than 790 kPa) typically employed in hydroalkylation processes also favor the competing reaction of benzene reduction to cyclohexane. As a result, cyclohexane and methyl cyclopentane selectivities of 5 to 20 wt % are commonly observed. The production of cyclohexane and methyl cyclopentane results in the loss of valuable benzene feed and reduces the level of benzene conversion (typically 40 to 60 wt %), which necessitate recycle of unreacted benzene.
Cyclohexylbenzene can be formed by benzene alkylation with cyclohexene using MCM-22 family catalyst. Cyclohexene can be produced by selective partial benzene hydrogenation using homogenous and heterogeneous catalyst as described in many patents and articles (EP 323192, JP 85-204370, U.S. Pat. No. 4,055,512, DE 2520430). The major by products formed in the benzene hydrogenation are similar to the hydroalkylation process (cyclohexane and methylcyclopentane) therefore the invention concept could be used to solve the problems associated with the benzene hydrogenation.
Unless removed, cyclohexane and methyl cyclopentane build up in recycle streams, thereby displacing benzene and increasing the production of undesirable by-products. However, the similar boiling points of benzene, cyclohexane and methyl cyclopentane render separation difficult with conventional distillation. The hydroalkylation process and the various end products (and unreacted benzene) are shown in the following:

Moreover, during dehydrogenation of the recycle stream (dehydrogenation of cyclohexane by-product to benzene), methyl cyclopentane by-product forms methyl cyclopentadiene, a very reactive precursor for coke formation and an accelerator of catalyst deactivation.

It would be desirable to have a process for hydroalkylating benzene affording enhanced conversion rates to cyclohexylbenzene. It would be further desirable to have a process wherein the build up of cyclohexane and methyl cyclopentane in recycle streams is reduced and the proportion of benzene therein is increased.